Poly(ethylene-2,6-naphthalene dicarboxylate), referred to as PEN, has been used in films, fibers, and molded objects. U.S. Pat. Nos. 3,546,008, 3,563,942, 3,734,874, and 3,779,993 disclose water-dispersible copolyester and polyesteramide compositions containing metal sulfonate groups. None of the references, however, use Poly(ethylene-2,6-naphthalene dicarboxylate) or mention Tg. U.S. Pat. No. 4,480,085 discloses compositions containing 1,8-naphthalene dicarboxylic acid and sodiosulfoisophthalic acid as the dicarboxylic acid component in combination with ethylene glycol. The sulfopolyester had a Tg of 73.degree. C. In contrast, the present inventors have determined that only certain naphthalenediyl isomers will yield polymers with Tg values above 89.degree. C.
U.S. Pat. No. 3,436,376, discloses polyesters synthesized from 2,6-naphthalene dicarboxylic acid, and ethylene glycol. The polyesters have Tg's of approximately 125.degree. C. Tg values of 88.degree. C. and 73.degree. C. were reported for the analogs where the ethylene glycol was replaced with 1,2-propane diol and 1,3-propane diol, respectively. The polyesters of U.S. Pat. No. 3,436,376 did not contain any sulfomonomer and were not water-dispersible or water-dissipatible.
U.S. Pat. No. 3,123,587, discloses compositions which are resistant to hydrolysis compared to 5-sulfoisophthalic acid modified poly(ethylene terephthalate) derived fibers. The compositions contain 1 to 5 mole % of 5-sulfoisophthalic acid with 75 mole % of the total acid component comprised of 2,6 or 2,7-naphthalenediyl units and 75 mole % of the total glycol component comprised of cyclohexanedimethanol. Improved hydrolytic stability was touted as an attribute of the invention. Thus, the polyesters are less likely to be water dispersible.
Copending commonly assigned U.S. Pat. Application Ser. No. 08/041,105 abandoned discloses water-dispersible sulfopolyesters having a Tg above 89.degree. C. which contain a dicarboxylic acid component of poly(ethylene-2,6-naphthalene dicarboxylate and a sulfomonomer, and a diol component containing at least 35 mole percent of selected diols. The sulfopolyesters of the copending application overcame the problems associated with prior patents which taught that increasing the glass transition temperature of the polyesters reduces the water-dispersibility of the polyesters. The copending application sulfopolyesters, however, posses high melt viscosities resulting in manufacturing problems.
Water-dispersible sulfopolyesters containing only isophthalate acid units or only terephthalic acid units tend to have Tg's less than 89.degree. C. with aliphatic and alicyclic diols, while analogous systems containing only naphthalene dicarboxylic acid units tend to have a maximum Tg around 130.degree. C., when common aliphatic and cycloaliphatic diols are used. Unexpectedly, the present inventors have determined that sulfopolyesters based on isophthalate or terephthalate units modified with naphthalene units not only maintain Tg's in the 89.degree. C. to 130.degree. C. range, but also exhibit melt viscosities lower than comparable systems containing only isophthalic acid or only naphthalene dicarboxylate units. It is desirable to have lower melt viscosities, since production process equipment is limited to a certain melt viscosity. Thus, lower melt viscosities will allow for higher molecular weights to be obtained for a given sulfopolyester.